Mercury, Mars, Jupiter

Professor: Astronomers help protect Earth with asteroid warnings.

This September you can see three bright planets in the morning sky just before sunrise, about 6:35 a.m. at the beginning of September.

About 45 minutes before sunrise, Mercury is visible 7 degrees (fourteen apparent moon diameters) above the east point on the horizon but you will need a clear horizon to see it. Mercury will look like a bright white star.

Be one of the few people who have seen Mercury in the sky. About 30 degrees above Mercury, but not as bright as Mercury, is blood-red Mars. To the right of Mars is the familiar constellation Orion. Don't mistake the bright red star Betelgeuse in Orion with Mars.

And almost 60 degrees above the southern horizon is brilliant Jupiter, the brightest of these three planets. Low on the southeast horizon is the very bright star Sirius. Don't confuse it with one of the planets.

Last Sept. 8 two small asteroids passed close to the Earth with very little warning. The asteroid designated 2010 RX30, about 15 meters long, came within 248,000 kilometers of the Earth. That is closer than the moon whose average distance is 384,000 kilometers.

The second slightly smaller asteroid, 2010 RF12 came within only 79,000 km. These asteroids were discovered only a few days before the close encounter. But this encounter was small potatoes compared to what might be coming in September 2082. A very large asteroid, 1999 RQ36, will have a nearly 1 in 1,000 chance of hitting Earth Sept. 24, 2082.

This asteroid is thought to be about 500 meters across. If it hits Earth a major disaster would result. NASA plans a visit to this asteroid and if it does threaten to impact the Earth there is time to deal with it.

The general strategy to deal with asteroids that threaten the Earth is not to blow them up because that probably wouldn't work, but to try to change their orbits. Whatever method you use to deal with these asteroids it is most important to identify them early, and that's a job for astronomers.

The harvest full moon falls Sept. 12. The harvest moon is the full moon closest to the autumn equinox. Occasionally it can fall in October. The full moon rises at sunset and normally the moon rises about 50 minutes later each night. But because of the position of the moon around the equinox, the harvest moon rises only about 30 minutes later each night, thus illuminating the fields in the evening for several nights.

Equinox means equal night and we usually say that on the day of the equinox the length of daylight equals the length of nighttime. However that is only approximately true. On my Minnesota Weatherguide calendar the sunrise and sunset times on the autumnal equinox, Sept. 23, are 7:01 a.m. and 7:09 p.m., so day and night aren't exactly equal in length.

In fact, the times of sunrise and sunset are closest to 12 hours apart on Sept. 26. This discrepancy is caused primarily by the effects of refraction. The solar disk you see rising is really the refracted image of the real disk just below the horizon.

The definition of sunrise and sunset also contributes to the discrepancy. Sunrise is when the leading edge of the apparent solar disk crosses the eastern horizon but sunset is when the following edge of the apparent solar disk disappears below the western horizon. Also keep in mind that the amount of refraction and the apparent size of the sun vary. Also only on the ocean do you have the horizon that the calculations refer to. All these effects make sunrise and sunset times approximate.

Just a year ago Sept. 30, the discovery of the first potentially Earth-like planet, Gliese 581g, was announced. Because this planet is neither too hot nor too cold for liquid water to exist, it has been called the Goldilocks planet.

Although some researchers have disputed the existence of this planet, other Earth-like planets have been discovered, and it is becoming increasingly likely that many Earth-like planets with life exist.

(Steve Kipp is a professor of astronomy at Minnesota State University, Mankato. Questions or comments about the monthly "Starline" column may be sent to Prof. Steve Kipp, 141 Trafton Science Center N, Mankato, MN, 56001, or to steven.kipp@mnsu.edu.)